This invention relates to a control apparatus of an electric vehicle having a relay between a battery and an electric motor.
In recent years, many electric vehicles in which driven wheels are driven using an electric motor have been developed. This kind of electric vehicle includes electric cars having only an electric motor as a motive power source and hybrid cars having an electric motor and an engine as motive power sources.
A drive circuit such as an invertor for producing a driving current for the electric motor is provided between the battery and the electric motor in this kind of electric vehicle, and a relay for supplying or cutting off current to/from the drive circuit is provided between the drive circuit and the battery. The relay for controlling the supply of current to the drive circuit is switched in correspondence with the state of an ignition switch (hereinafter abbreviated to switch), and when the switch is turned ON by a driver the relay is switched to a connecting state and current is supplied to the drive circuit through the relay, and when the switch is turned OFF by the driver the relay is switched to a disconnecting state and current to the drive circuit through the relay is cut off.
In electric vehicles of recent years, to increase the motive power of the vehicle, the electrical power supplied to the electric motor has been tending to increase. Consequently, a large current has also been supplied to the relay, and when inadvertently the switch is turned OFF and the relay is switched to its disconnecting state, there has been a risk of a spark being produced across the contacts of the relay and of welding failure of the relay contacts occurring.
When this kind of welding failure occurs in the relay, because the output current from the battery cannot be cut off, it becomes difficult to ensure the safety of the vehicle. To overcome this, electric vehicles have been developed (see for example JP-A-10-144194) in which, when a disconnection signal has been outputted to the relay, by the voltage of a power supply circuit being measured, a diagnosis is made of whether or not the relay has disconnected normally, that is, whether or not there has been a welding failure. In this electric vehicle, when a welding failure of the relay has been detected, safety of the vehicle is ensured by travel prohibition processing, charge prohibition processing and alarm processing being carried out.
However, even if after a welding failure of the relay contacts is diagnosed safety of the vehicle is secured by these processes being carried out, because as a result of the welding failure the vehicle becomes unable to travel, a control apparatus capable of preventing welding failure before it occurs has been needed. And from the point of view of repair cost also, it is desirable for welding failure of the relay contacts to be prevented before it occurs.
Also, even if welding failure of the relay contacts does not occur, whenever the relay is switched to its disconnecting state by the switch being inadvertently turned OFF, there is a risk of the drive circuit being destroyed. That is, in a vehicle using a permanent magnet type motor, because an induced voltage arises along with rotation of the motor, when the relay is disconnected while the electric motor is rotating and the battery is thus cut off from the electric motor, there is a risk of the induced voltage from the electric motor being applied locally.